The increased attention paid to global warming in recent decades has led to increased research in the field of power generation. Different measures for fighting against the undesirable effects of global warming have been proposed. One such measure is carbon capture and storage (CCS), which has widely been considered a mid-to-long term mitigating measure against the emission of CO2. CCS has the potential to be both valuable and environmental, and this can be achieved if CO2 can be utilized in industrial applications after it has been captured. For example, CO2-EOR (Enhanced Oil Recovery by injecting CO2 into oil reservoirs) is a potential industrial usage of CO2 that increases petroleum production. This process has been commercially used for approximately 40 years, typically utilizing CO2 from natural resources, and its feasibility has been well recognized in terms of transportation and injection of CO2. Nevertheless, the biggest problem with CCS lies not with the potential industrial uses of the captured CO2, but rather with the costliness of current CCS processes.
Combustion is a commonly used reaction in the field of power generation. Several carbon capturing techniques exist for capturing CO2 from a combustion unit, including post-treatment, O2/CO2 firing (oxyfuel), and CO-shift. These CO2 capture techniques all suffer from the fact that very significant gas separation steps are needed. Moreover, these gas separation steps involve very significant operational costs as well as large energy penalties, estimated in the order of about 10 percentage points of plant efficiency, leading to a substantial increase—25 to 30%—in fuel consumption and plant size. Gas separation technology is generally a mature technology, and no major technology breakthroughs in this area are foreseen.
This is in great contrast to chemical looping combustion (CLC) where no gas separation is needed. CLC is a specific type of combustion reaction that was originally created in the 1950s to produce CO2, but recently it has received increased attention as a potential CO2 capturing process. In a CLC process, an oxygen carrier acts as an intermediate transporter of oxygen between air and fuel, and thus the air and the fuel are prevented from directly contacting one another. As a result, the exhaust gas stream ideally consists of CO2 and H2O only, and the CO2 is readily available after condensation of H2O. Thus the energy requirement of gas-gas-separation is avoided.
In general, the overall heat of a CLC process will be the sum of the two heat states, exothermic during oxidation and endothermic during reduction, which is equivalent to the heat released in a convention combustion reaction. Accordingly, one advantage of the CLC process is that minimal extra energy is required to capture CO2 while still maintaining a combustion efficiency similar to direct combustion processes. More precisely, there is minimal energy penalty for CO2 capturing in a CLC process, estimated at only 2-3% efficiency lost. Additionally, NOx formation is reduced in the CLC process compared with direct combustion processes as the oxidation reaction occurs in the air reactor in the absence of fuel and at a temperature of less than 1200° C.—above which NOx formation increases considerably. Thus, the lack of NOx formation makes CO2 capturing in CLC processes less costly compared with other combustion methods because CO2 does not need to be separated from the NOx gas prior to capture. Overall, CLC is one of the few technologies today where a significant breakthrough could be envisaged for avoiding the large costs and energy penalty of gas separation in CO2 capture.
A key factor for the CLC technology development is the selection of an oxygen carrier. Suitable oxygen-carriers must show high reaction rates and oxygen transport capacity, complete fuel conversion to CO2 and H2O, negligible carbon deposition, avoidance of agglomeration, sufficient durability, and good chemical performance. These properties must be maintained during several reduction and oxidation cycles. In addition, the cost of the oxygen-carrier is also important.
In a typical chemical looping combustion process, a solid metal oxide oxygen carrier is used to oxidize the fuel stream in a fuel reactor. Transition metal oxides such as nickel, copper, cobalt, iron, and manganese are good oxygen carrier candidates because of their favorable reductive and oxidative thermodynamic properties. Still, the effective use of metal oxides as oxygen carriers can make CLC processes costly. For instance, U.S. Pat. No. 5,447,024 claims as the active mass the use of redox pair NiO/Ni combined with the binder type yttriated zirconia in order to improve the mechanical strength and the reactivity of the particles. Using binder's type yttrium-zirconia increases the cost of the metal oxide and consequently the cost of a CLC process.
Black powder is regenerative and is formed inside natural gas pipelines as a result of corrosion of the internal walls of the pipeline. Black powder forms through chemical reactions of iron (Fe) in ferrous pipeline steel with condensed water containing oxygen, CO2, and other gases. FIG. 1 shows an exemplary pipe 10 that has an outer surface and an opposing inner surface 12. Black powder 20 is shown collecting along the inner surface 12 of the pipe 10.
Black powder is mainly composed of iron hydroxides, iron oxides, and iron carbonates. As used herein including in the present claims, black powder refers to the residue (material) that is formed along inner surface of pipelines as a natural waste product as a result of corrosion and comprises a metal oxide. Black powder can also be collected from upstream filters employed in gas refineries.
For many years, pipeline companies have observed the presence of black powder and its effects, but have viewed it only as a nuisance and therefore have done little to understand it and use it. Instead, pipeline companies have primarily sought ways of removing the black powder from the pipelines. There are several methods to remove the black powder, such as separators and cyclones, where the black powder-laden gas passes through these devices and the black powder particles are physically knocked out of the gas stream. Specifically, the black powder particles are removed from the gas stream and attach to the walls of the device (e.g., separator, cyclone) where they fall and are collected at the bottom in a collection media.
Pipeline companies have yet to find a beneficial use for the black powder. Throughout the world, black powder from gas pipeline exists in large amounts, and is thus readily available at a very low cost due to its perceived lack of value. Today, black powder is generally discarded as waste.
Overall, there is a need for efficient CO2 capture in the field of power generation in light of growing concerns regarding global warming. Further, there is a need to reduce the cost of current CLC processes, and in particular, reduce the cost of oxygen carriers utilized in CLC processes. Lastly, there is a need to utilize the black powder waste formed in gas pipelines.